V43F-01 13:45h
Geophysical features of hydrothermal system in Suiyo Seamount, Izu-Ogasawara Arc, Western Pacific
Surface geophysical, airgun-OBS (ocean bottom seismograph), and deep-towed surveys by R/V Kairei KR01-15 cruise, and ten OBS array observation from Aug. 5 to Oct. 9, 2002 were conducted to characterize geophysical features of a hydrothermal system in Suiyo Seamount as a part of Archaean Park Project. Geomagnetic results from surface and deep towed magnetometers suggest 1) the seamount was formed during Brunhes Epoch (after 0.78Myr), 2) low magnetization area exists on its summit and is limited only in eastern side with its diameter of 600 meters. 2-D ray tracing results, using four OBS data with a GI gun across the seamount in NE-SW direction, indicate that P wave velocity of 2.2-4.2 km/s (2km thickness) locates all over the observation line. Two-month OBS observation reveals the seismic activity, which is characterized by dominance of earthquakes with S-P times of 1-2 sec. The hypocenter determination indicates that the locations of these earthquakes are focused within 3 km in diameter just beneath the Suiyo volcanic cone at a depth of 3-7 km. We propose a model to explain these geophysical observations; the former main conduit of Suiyo Seamount is in the middle of cooling after the last eruption, and its heat drives the hydrothermal system in Suiyo Seamount. The low magnetization area probably results from high alteration of the rock through the hydrothermal activity. The area with P wave velocity of 2.2-4.2 km/s (2km thickness) suggests higher porosity area where a major hydrothermal circulation exists. The hypocenter locations below a depth of 3 km probably correspond to the locations close to the former main conduit boundary where high thermal stress is expected due to changes in temperature and high thermal gradient. The area without seismicity (less than 3 km in depth) has probably released the thermal stress already through the hydrothermal circulation. This scenario is also supported by our numerical simulation; a magma injection in the conduit with its diameter of 1 km maintains its heat during a few thousands years, although the surface hydrothermal activity cools the uppermost part of the conduit.
V43F-02 14:00h
Discharge/recharge boundary of hydrothermal venting detected by closely-spaced geothermal measuremsnts
Hydrothermal activity within the caldera of Suiyo Seamount was investigated in detail using HOV/ROV, and by deep-tow imagery and seismic surveys. Hydrothermal regime in the Suiyo-seamount is characterized by a geochemically uniform fluid, shallow reservoir depth, very permeable seafloor, and venting without creating big chimneys. Detailed heat flow surveys were carried out through five research cruises. Heat flow is highest ($>$ 10 W/m$^{2}$) within the active area. On the western and southern periphery, some heat flow is lower than 0.3 W/m$^{2}$, only several tens of meters away from the venting area. To the east, heat flow is uniform around 4 W/m$^{2}$, indicating a uniform reservoir overlain by impermeable layer. High temperature zones below seafloor, with non-linear temperature profiles, are identified next to the isolated vents indicating a very local circulation. We believe that we obtained several lines of direct evidence for discharge and recharge activity and their interface near the vents, constrained by the precipitation structure around the vents. Hydrothermal regime at the Suiyo site consists from a 100-m scale, stable circulation to a 1-m scale local patterns. This multiple-scale circulation pattern, also identified at other hydrothermal sites, would be constrained by the distribution of heat source and hydrological properties near the seafloor.
V43F-03 INVITED 14:15h
Lessons from Suiyo Seamount studies, for understanding extreme (ancient?) microbial ecosystems in the deep-sea hydrothermal fields
Deep-sea hydrothermal ecosystems are driven with various geo-thermally modified, mainly reduced, compounds delivered from extremely hot subsurface environments. To date, several unique microbes including thermophilic archaeons have been isolated from/around vent chimneys. However, there is little information about microbes in over-vent and sub-vent fields. Here, we report several new findings on microbial diversity and ecology of the Suiyo Seamount that locates on the Izu-Bonin Arc in the northwest Pacific Ocean, as a result of the Japanese Archaean Park project, with special concern to the sub-vent biosphere. At first, we succeeded to reveal a very unique microbial ecosystem in hydrothermal plume reserved within the outer rim of the seamount crater, that is, it consisted of almost all metabolically active microbes belonged to only two Bacteria phylotypes, probably of sulfur oxidizers. In the center of the caldera seafloor (ca. 1,388-m deep) consisted mainly of whitish sands and pumices, we found many small chimneys (ca. 5-10 cm) and bivalve colonies distributed looking like gray to black patches. These geo/ecological features of the seafloor were supposed to be from a complex mixing of hydrothermal venting and strong water current near the seafloor. Through quantitative FISH analysis for various environmental samples, one of the two representative groups in the plume was assessed to be from some of the bivalve colonies. Using the Benthic Multi-coring System (BMS), total 10 points were drilled and 6 boreholes were maintained with stainless or titanium casing pipes. In the following submersible surveys, newly developed catheter- and column-type in situ growth chambers were deployed in and on the boreholes, respectively, for collecting indigenous sub-vent microbes. Finally, we succeeded to detect several new phylotypes of microbes in these chamber samples, e.g., within epsilon-Proteobacteria, a photosynthetic group of alpha-Proteobacteria, and hyperthermophile-related Euryarchaea. By the FISH analysis, however, some specific members of Bacteria that differed from those in the chamber samples were occasionally abundant in hot vent fluids. In clone library analysis of column-type chamber samples, we also found very unique vertical profiles in the community of Archaea, i.e., rich in uncultivable Marine Group I & II members in the upper and middle columns situated at a warm vent site, while heterotrophic thermophiles in the middle and bottom. In a bottom column sample from a hot vent site, hyperthermophilic anaerobes were detected. From these results, we will propose a vertical profile model for the sub-vent Archaea community. From geophysical, geochemical and geological surveys, this sub-vent ecosystem is supposed to be restricted in a shallow subsurface region. Whether these unique ecosystems are general in hydrothermal fields over the sea or specific only to this submarine volcano will be discussed.
V43F-04 14:30h
Microbially Produced Enrichment of CO and N$_{2}$O in Hydrothermal Plumes
Effluent hydrothermal plumes are enriched with some components (such as CH$_{4}$, Mn, $^{3}$He, etc.) from surrounding seawater, because of extreme enrichment of these components in seafloor venting hydrothermal fluids from seawater. In recent studies, however, we found enrichment with CO and/or N$_{2}$O in hydrothermal plumes, while hydrothermal fluids exhibit comparable concentrations with seawater for the components. The enrichment with CO and/or N$_{2}$O in a hydrothermal plume reflects in situ microbial activities that have been stimulated by some primary components that derived from hydrothermal fluids. We took effluent hydrothermal plume water samples in and around the water column of Suiyo seamount (ca. 1,380m depth and ca. 2km diameter), Izu-Bonin arc, to determine concentrations and stable carbon isotopic compositions of carbon monoxide (CO) and methane (CH$_{4}$) in the plume. We also sampled venting fluids using manned submersible Shinkai 2000 to compare the chemical composition and isotopic composition of CO and CH$_{4}$ with those in the hydrothermal plume. We detected strong CH$_{4}$ anomalies not only on the water columns within the caldera, but also on those at outsides of the caldera at the depth of ca. 1,100-1,150m, the sill depth of the caldera wall. Within the plume, we detected significant CO enrichment. The stable carbon isotopic compositions of CO in the plume, however, exhibit highly $^{13}$C-depleted values (-110 to -60 \permil PDB) compared with those in seafloor venting hydrothermal fluids (around -30 \permil PDB). Besides, the plume samples exhibit higher CO/CH$_{4}$ ratio along with the distance from the fluid venting site. We conclude that some microbial activities result in CO production within the hydrothermal plume. Partial decompositions of CH$_{3}$OH produced during microbial CH$_{4}$ oxidation might be a possible origin for the CO in hydrothermal plume. This CO enrichment in deep seawater can be a good tracer for seafloor hydrothermal activities, especially detecting plumes located on rather distant place from venting site. Besides, the secondary chemical anomalies can be a tracer for quantifying in situ microbial activities in hydrothermal plumes.
V43F-05 14:45h
Active Volcanic and Hydrothermal Processes at NW Rota-1 Submarine Volcano: Mariana Volcanic Arc
Dives with the remotely operated vehicle ROPOS in March/April 2004 documented a volcanic eruption at NW Rota-1, a submarine volcano of basaltic composition located at 14\deg 36.0'N, 144\deg 46.5'E lying 65 km northwest of Rota Island in the Commonwealth of the Northern Mariana Islands. The site was chosen as a dive target because of the of the high concentrations of H2S and alunite in the hydrothermal plume overlying its summit in February 2003. The summit of the volcano is composed of curvilinear volcanic ridge oriented NW-SE bounded by NE-SW trending normal faults. Lavas collected on the upper part of the edifice are primitive to moderately fractionated basalts (Mg# = 51-66). The eruptive activity is occurring within a small crater (Brimstone Pit) located on the upper south flank of the volcano at 550 m, about 30 m below the summit. The crater is approximately 15 m wide and at least 20 meters deep. The ROPOS's cameras observed billowing clouds of sulfur-rich fluid rising out of the crater, punctuated by frequent bursts of several minutes duration that entrained glassy volcanic ejecta up to at least 2 cm in diameter. ROPOS recorded a temperature of 38\degC within the plume. The volcanic activity had substantial temporal variability on the scale of minutes. ROPOS was sometimes completely enveloped by the plume while on the rim of the crater, and its surfaces were coated with large sulfur droplets. Black glassy fragments were entrained in the plume up to least 50 m above the crater and deposits of this material were on ledges and tops of outcrops up to several hundred meters from Brimstone Pit. The pit crater fluids have an extremely high content of particulate sulfur and extremely acidic, with pH around 2.0. This strongly implicates magmatic degassing of SO$_{2}$ and disproportionation into elemental S and sulfuric acid. Diffuse venting of clear fluids was also present on the summit of the volcano, with temperatures exceeding 100\degC in volcaniclastic sands adjacent to Brimstone Pit. There was also a distinct layer of turbid water on the flanks of the volcano below 700 m that is almost surely related to the eruptive activity. The layer is probably caused by resuspension of material from frequent slope failure due to episodic deposition of ejecta downslope from Brimstone Pit. The vent macrofauna on the summit of NWRota-1 consists of predominantly of shrimps and crabs with a notable dearth of sessile organisms. The unstable slopes and rain of sulfur and volcanic particulates would discourage colonies of less mobile organisms (e.g., mussels tubeworms) found at other Mariana hydrothermal sites.
V43F-06 15:00h
The Geologic Setting of Hydrothermal Vents at Mariana Arc Submarine Volcanoes: High-Resolution Bathymetry and ROV Observations
Remotely operated vehicle (ROV) dives were made at 7 submarine volcanoes between $14-23\deg$N in the Mariana Arc in April 2004 with the ROPOS ROV. Six of these volcanoes were known to be hydrothermally active from CTD data collected during a previous expedition in March 2003: NW Rota-1, E Diamante, NW Eifuku, Daikoku, Kasuga-2, and Maug, a partly submerged caldera. The physical setting of hydrothermal venting varies widely from volcano to volcano. High-resolution bathymetric surveys of the summits of NW Rota-1 and NW Eifuku volcanoes were conducted with an Imagenex scanning sonar mounted on ROPOS. Near bottom observations during ROPOS dives were recorded with digital video and a digital still camera and the dives were navigated acoustically from the R/V Thompson using an ultra-short baseline system. The mapping and dive observations reveal the following: (1) The summits of some volcanoes have pervasive diffuse venting (NW Rota-1, Daikoku, NW Eifuku) suggesting that hydrothermal fluids are able to circulate freely within a permeable edifice. At other volcanoes, the hydrothermal venting is more localized (Kasuga-2, Maug, E Diamante), suggesting more restricted permeability pathways. (2) Some volcanoes have both focused venting at depth and diffuse venting near the summit (E Diamante, NW Eifuku). Where the hydrothermal vents are focused, fluid flow appears to be localized by massive lava outcrops that form steep cliffs and ridges, or by subsurface structures such as dikes. High-temperature ($240\deg$C) venting was only observed at E Diamante volcano, where the "Black Forest" vent field is located on the side of a constructional cone near the middle of E Diamante caldera at a depth of 350 m. On the side of an adjacent shallower cone, the venting style changed to diffuse discharge and it extended all the way up into the photic zone (167 m). At NW Eifuku, the pattern of both deep-focused and shallow-diffuse venting is repeated. "Champagne vent" is located at 1607 m, ~150 m below the summit, and is characterized by focused flow of CO2-rich fluids, whereas the summit has extensive areas of diffuse venting and is covered with thick bacterial mats. (3) Some of the most remarkable vent sites are deep, narrow volcanic craters at NW Rota-1 and Daikoku volcanoes. The crater at NW Rota-1 volcano (named "Brimstone Pit") is 15-m wide, 20-m deep, funnel shaped, and was actively erupting ash, lapilli, and molten sulfur. The rim of Brimstone Pit is composed of welded spatter and is located at 550 m depth, about 30 m below the summit. Other diffuse hydrothermal sites at NW Rota-1 are located along the rocky summit ridge. At Daikoku volcano, an extraordinary crater emitting cloudy hydrothermal fluid was found at 375 m depth on the north shoulder of the volcano, about 75 m below the summit. This crater was at least 135 m deep and had a remarkably cylindrical cross-section with a diameter of ~50 m. ROPOS descended 75 m into the crater and was still at least 60 m above the bottom, according to the altimeter, when we were forced to cease operations due to weather. In addition, diffuse hydrothermal fluids seep from large areas of the summit and upper slopes of Daikoku.
V43F-07 15:15h
Petrology and Geochemistry of Igneous Rocks collected in Association with ROV Investigations of Three Hydrothermal Sites in the Mariana Arc: NW Rota-1, E. Diamante, and NW Eifuku
Dives with the remotely operated vehicle ROPOS in March/April 2004 discovered magmatic degassing and hydrothermal activity at three important sites in the submarine Mariana arc: NW Rota-1 (14°36'N, 144°46'E), E. Diamante (15°56'N, 145°41'E), and NW Eifuku (21°29'N, 144°03'E). The first two volcanoes are part of the Mariana Southern Seamount Province whereas NW Eifuku is part of the Northern Seamount Province. These hydrothermal sites are associated with the summit regions (NW Rota-1, NW Eifuku) or the resurgent dome of a caldera (E. Diamante). NW Rota-1 appears to be volcanically active, whereas E. Diamante is thought to be extinct. NW Eifuku has not been sampled previously, but vigorous CO2 bubbling from the Champagne site indicates degassing of magma. Lavas previously analyzed from the edifice flanks of E. Diamante are fractionated basalts and basaltic andesites. All samples belong to a medium-K calcalkaline suite, similar to most Mariana arc lavas. Major element compositions for these three suites span the entire fractionation range observed for IBM arc lavas. NW Rota-1 lavas are vesicular OL-CPX-plag porphyritic basalts and basaltic andesites (51-54 per cent silica) and NW Eifuku samples are vesicular OL-CPX-PLAG basalts (50-51per cent silica). E. Diamante samples are QZ-PLAG porphyritic rhyodacites (66-75 per cent silica); we cannot be sure whether these samples represent lavas or hypabyssal intrusive rocks. Abundant vesicles in NW-Rota-1 basalts is expected, given eruption depths of 500m, but abundant vesicles in NW Eifuku lavas, erupted at 1600m, is more surprising. NW Rota-1 (Mg# = 57 to 66) and NW Eifuku basalts (Mg# = 58-66) are unusually primitive for IBM arc lavas. These basalts are approximately saturated in silica. NW Rota-1 has Na6 = 2.5 whereas NW Eifuku has Na6 = 2.1, indicating significantly higher degrees of melting. Studies of mineralogic, trace element, and isotopic compositions are still in progress.
V43F-08 15:30h
Liquid Carbon Dioxide Venting at the Champagne Hydrothermal Site, NW Eifuku Volcano, Mariana Arc
In March/April 2004, submersible dives with the remotely-operated vehicle ROPOS discovered an unusual CO$_{2}$-rich hydrothermal system near the summit of NW Eifuku, a submarine volcano located at 21.49$\deg$N, 144.04$\deg$E in the northern Mariana Arc. Although several sites of hydrothermal discharge were located on NW Eifuku, the most intense venting was found at 1600-m depth at the Champagne site, slightly west of the volcano summit. The Champagne site was found to be discharging two distinct fluids into the ocean: a) several small white chimneys were emitting milky 103$\deg$C gas-rich hydrothermal fluid with at least millimolar levels of H$_{2}$S and b) cold ($<$ 4$\deg$C) droplets coated with a milky skin were rising slowly from the sediment. These droplets were later determined to consist mainly of liquid CO$_{2}$, with H$_{2}$S as a probable secondary component. The droplets were sticky, and did not tend to coalesce into larger droplets, even though they adhered to the ROV like clumps of grapes. The film coating the droplets was assumed to be CO$_{2}$ hydrate (or clathrate) which is known to form whenever liquid CO$_{2}$ contacts water under these P,T conditions. Samples of the 103$\deg$C hydrothermal fluids were collected in special gas-tight titanium sampling bottles that were able to withstand the high internal pressures created by the dissolved gases. The Champagne hydrothermal fluids contained a surprising 2.3 moles/kg of CO$_{2}$, an order of magnitude higher than any CO$_{2}$ values previously reported for submarine hydrothermal fluids. The overall gas composition was 87% CO$_{2}$, $<$ 0.1% CH$_{4}$, $<$ 2 ppm H$_{2}$, 0.012 mM/kg $^{4}$He, with the remaining 13% (322 mM/kg) assumed to be sulfur gases (H$_{2}$S, SO$_{2}$, etc.). (Additional analyses planned will confirm the speciation of this sulfur gas component). The helium had R/R$_{A}$ = 7.3, typical of subduction zone systems (R = $^{3}$He/$^{4}$He and R$_{A}$ = R$_{air}$). Isotopic analysis of the CO$_{2}$ yielded $\delta$$^{13}$C = -1.75 $\permil$, much heavier than the -6.0 $\permil$ typical for carbon in MOR vent fluids. The C/$^{3}$He ratio was ~2.2 x 10$^{10}$, an order of magnitude higher than the average value of 2 x 10$^{9}$ found in MOR vent fluids. The $\delta$$^{13}$C and C/$^{3}$He values suggest a substantial contribution to the carbon from subducted carbonates rather than mantle carbon. The Champagne site is only the second locality where liquid CO$_{2}$ has been observed venting into the deep sea (the other reported location is in the Okinawa Trough, see Sakai et al., 1990). Because of the presence of liquid CO$_{2}$ in proximity to hydrothermal organisms, the Champagne site may prove to be a valuable natural laboratory for studying the effects of high CO$_{2}$ concentrations on marine ecosystems.